Resolve 18 merge conflicts in serial subsystem. Keep branch's design:
- Separate handler functions per subcommand (handleGet/handleSet/handleClear)
- Named subcommand registration via CommandGroup
- getCommandEntry() for intelligent subcommand matching
- CommandGroup in separate files (not duplicated in CommandEntry.h)
Adopt from develop:
- OS_SERIAL_PRINT/PRINTF/PRINTLN macros (Arduino→ESP-IDF migration)
- USB serial support (#if ARDUINO_USB_MODE)
- TinyVec usage for rawconfig
- LedTestHandler
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Split apart command handlers and fix command handler selection logic